The Tasman Abyssal Plain, or the Eastern Australian Abyss, is an abyssal plain located off the coast of southeast Australia in the Tasman Sea. Extending from southern Queensland to Tasmania, the large submarine plain is distinguished by its delicately sloping terrain and abundant marine biodiversity. The plain's deep sea floor is notable for its important sedimentary deposits and exceptional geological formations, whereby showcasing the earth's physical history. Also known as the East Australian Basin and Tasman Basin, it is characterized by an average depth between 4,000 m (13,000 ft) and 5,000 m (16,000 ft), with a diverse, though scarcely-explored, ecosystem. A world-first observation of the largely-unexplored abyssal plain took place in 2017, where it created high quality seafloor mapping of Australia's continental edge from southern Queensland to Tasmania. The sonar observation revealed that the continental slope to the eastern Australian abyss was more convoluted and craggy than expected, with the edge being engraved by multiple canyons, sediment slumps, precipices and many areas of exposed rock. The majority of abyssal species discovered have extensive geographic and bathymetric distributions. Polychaetes, which are marine worms, were the leading members of the deep-sea macrobenthos, and therefore were valuable organisms for zoogeographical studies.
Geology The eastern Australian abyss was formed around 90–52 million years ago during the tectonic breaking of Australian and New Zealand continental masses. A south-north expanding ridge previously existed at the plain's centre. Thereafter, the north–south string of Tasmantid seamounts were formed along the Lord Howe Rise by the Australian plate shifting over a crustal centre from 33.5 to 6.4 million years ago. The current basement depths and the sediment thickness hint a Cretaceous age. Today, the abyssal plain stretches from the Chesterfield and Marion plateaus near Queensland's coastal areas, in the north, (at 24°S), to Tasmania's south (48°S), before heading into the Southern Ocean. The upper continental slope off eastern Australia is largely arched from the shelf break down to 1,500 m (4,900 ft). At mid slope depths, around 3,000 m (9,800 ft), it is more indented, with the higher section occasionally forming an escarpment. Finally, on the lower slope, a series of tread-like rocky racks can be found down to the abyssal plain 4,500 m (14,800 ft) to 5,000 m (16,000 ft). The periphery is repeatedly divided by canyons, some of which begin at shelf depths and depressions due to failing of the incline. The International Ocean Discovery Program had a site located 945 km (587 mi) east of Australia, a site that was on an oceanic crust of Late Cretaceous that was formed 74–83 million years ago. Local seismic reflection data indicate a thick series of sediments, around 800 m (2,600 ft), that are deformed by reverse faults and folds. The basin has an average maximum depth of approximately 5,000 m (16,000 ft), although several smaller depressions extend deeper, with the greatest recorded depth of 5,943 m (19,498 ft) located just northwest of Sydney. At the southern end of the eastern Australian abyss, the basin is bounded by the Indo-Antarctic Ridge, where depths are generally less than 3,500 m (11,500 ft), limiting the northward movement of deep Antarctic waters. To the north, the eastern Australian abyss is connected to the Coral Sea by a ridge with depths generally shallower than 3,000 m (9,800 ft). The eastern Australian continental shelf is comparatively narrow compared with the rest of the continent, since the shelf breaks just 15 km (9.3 mi) away from the coast, where the beginning of the abyssal plain may be as proximate as 60 km (37 mi) from the coast. Geomorphological features in the eastern margin include plateaus, basins, terraces, holes, and submarine canyons, with one prominent example being Bass Canyon.
Oceanography
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